An apparatus of the kind in question which is described in DE-A-4 406 046 comprises a velocity measuring means for measuring the velocity of the powder/gas mixture in the feed pipe, a mass measuring means for measuring the powder mass per volume unit in a section of the feed pipe, the mass measuring means comprising a microwave resonator as well as means for detecting a change in the resonant frequency and/or the microwave amplitude of the microwave resonator and for deriving the powder mass in the feed pipe section from the resonant frequency and/or the microwave amplitude detected, and a calculating means for calculating the powder mass flow based on the velocity measured, the powder mass per volume unit measured, and the dimensions of the feed pipe. DE-A-4 406 046 likewise discloses a method of measuring a powder mass flow in a powder/gas mixture while the mixture is being conveyed through a feed pipe. With this method, the velocity of the powder/gas mixture is measured in the feed pipe, the powder mass per volume unit is measured in a section of the feed pipe by detecting a displacement of the resonant frequency in a microwave resonator disposed at the feed pipe, and the powder mass flow is calculated based on the velocity measured, the powder mass per volume unit measured, and previously known dimensions of the feed pipe.
The coating powder to be supplied to powder coating devices, in electrostatic coating systems, for example, is fluidized, i.e. the powder is intermixed with a gas, preferably with air in order to form a powder/air mixture which then is fed to the coating devices through feed pipes or tubes. In this context, it may become necessary to find out what quantity of powder per time unit actually is conveyed or delivered, in other words to determine the powder mass flow (g/min) during operation of the coating equipment, so as to be able to adjust the powder mass flow to a desired value or to maintain an set value.
In known powder coating equipment, the air pressure used to supply powder by means of injectors or venturi nozzles has been measured with the aid of pressure gauges providing values in bar, or the air quantities required for such supply have been determined in m.sup.3 /h.
However, measurements of the air pressures and air quantities offer no more than an indirect and unreliable indication of the actual output of powder. For instance, when a powder reservoir is empty, supply pressures or air quantities continue to be indicated although powder can no longer be delivered.
The above mentioned DE-A-4 406 046 suggests that two parameters be measured to determine the powder mass flow, namely:
the velocity V of the powder/air mixture in the feed pipe in m/sec and PA1 the powder mass per volume unit M.sub.V in the feed pipe in g/cm.sup.3.
Based on these two parameters V and M.sub.V, in combination with the dimensions of the feed pipe which are assumed to be known, especially the cross section Q of the feed pipe, the resulting powder mass flow fed and delivered, i.e. the powder quantity per time unit M.sub.T in g/min is given as follows: EQU M.sub.T g/min!=V m/sec!*M.sub.V g/cm.sup.3 !*Qcm.sup.2 !*6000.
According to the publication cited above the velocity is measured by means of two spaced apart measuring electrodes which sense charge fluctuations at the feed pipe generated by the powder/gas mixture being conveyed. The powder mass per volume unit is measured by means of a microwave resonator which senses variations of the dielectric constant and/or the microwave absorption in a resonant volume of the feed pipe by way of a shift or displacement of the resonant frequency or a change of the microwave amplitude in the resonator.
EP-A-297 309 discloses a method of and an apparatus for measuring and controlling the amount of powder in a powder coating system. Here powder quantities supplied per unit time to a spray coating machine are measured. The method provides for measuring the density of the powder in the powder/gas flow by a beam measuring device. The gas quantity per unit time is determined, and the amount of powder conveyed per unit time is established on the basis of the following equation c=a.multidot.b.
U.S. Pat. No. 4,807,480 describes a method of measuring the flow rate of an powder/air mixture in which the flow velocity of the mixture is detected by sensors which are spaced from each other and which sense charge fluctuations in the powder/air mixture. To accomplish this, a corona needle electrode injects a charge into the powder/air mixture and the sensors detect the charge.
JP-A-3-125924 discloses a flowmeter for a powder flow. Here, a tracer material is introduced into a stream of powder and two sets of microwave transmitters and receivers are used to detect the powder mass flow.